Geodetic surveying with time synchronization

ABSTRACT

The present disclosure provides a method for determining a direction to a geodetic target from a geodetic instrument. The method includes emitting an optical pulse from the geodetic target, capturing a first image and a second image of the geodetic target using a camera arranged at the geodetic instrument, obtaining a difference image between the first image and the second image, and determining a direction to the geodetic target from the geodetic instrument based on the position of the optical pulse in the difference image. The method further includes synchronizing the geodetic instrument and the geodetic target for emitting the optical pulse concurrently with the capturing of the first image and nonconcurrently with the capturing of the second image. The present disclosure also provides a geodetic instrument, a geodetic target and a geodetic surveying system.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of International ApplicationNo. PCT/EP2017/053252, filed Feb. 14, 2017, the contents of which areincorporated herein by reference in their entirety for all purposes.

TECHNICAL FIELD

The present disclosure generally relates to the field of land surveying.In particular, the present disclosure relates to a method fordetermining a direction to a geodetic target from a geodetic instrument.The present disclosure relates also to a geodetic instrument, a geodetictarget and a geodetic surveying system.

BACKGROUND

A geodetic instrument, such as a total station or a theodolite, may beused to determine directions to and/or positions of one or more geodetictargets. The targets may for example be used for stake outs onconstruction sites, or positioned on construction vehicles operating onthe construction site in order to guide these vehicles in theiroperation.

On larger construction sites, multiple targets may be present at thesame time, and/or the geodetic targets may be located at rather longdistances from the geodetic instrument. Therefore, there is a need forimproved geodetic instruments, geodetic targets, and methods foroperating these to accurately and efficiently determine directions toand/or positions of geodetic targets.

SUMMARY

To at least partially fulfill the above requirements, the presentdisclosure seeks to provide at least one improved method, an improvedgeodetic instrument and an improved geodetic target for determining adirection to the geodetic target from the geodetic instrument.

To achieve this, methods, a geodetic instrument and a geodetic target asdefined in the independent claims are provided. Further embodiments ofthe present disclosure are provided in the dependent claims.

According to a first aspect, a method for determining a direction to ageodetic target from a geodetic instrument is provided. In the method,an optical pulse may be emitted towards the geodetic instrument from thegeodetic target, and a first image and a second image of the geodetictarget may be captured using at least one imaging device arranged at thegeodetic instrument. The geodetic instrument and the geodetic target maybe synchronized for emitting the optical pulse concurrently with thecapturing of the first image, and for emitting the optical pulsenonconcurrently with the capturing of the second image. A differenceimage between the first image and the second image may be obtained, andthe direction to the geodetic target from the geodetic instrument may bedetermined based on a position of the optical pulse in the differenceimage.

According to a second aspect, a method implemented by a geodeticinstrument for determining a direction to a geodetic target configuredto identify itself by emitting an optical pulse is provided. In themethod, a first image and a second image of the geodetic target may becaptured using at least one imaging device arranged at the geodeticinstrument. The geodetic instrument and the geodetic target may besynchronized for capturing the first image concurrently with emission ofthe optical pulse by the geodetic target, and for capturing the secondimage nonconcurrently with emission of the optical pulse by the geodetictarget. A difference image between the first image and the second imagemay be obtained, and the direction to the geodetic target from thegeodetic instrument may be determined based on a position of the opticalpulse emitted by the geodetic target in the difference image.

According to a third aspect, a geodetic instrument is provided. Thegeodetic instrument may include a processing unit that may be configuredto cause at least one imaging device to capture at least a first imageand a second image of a geodetic target configured to identify itself byemitting an optical pulse. The processing unit may be further configuredto synchronize the at least one imaging device to capture the firstimage concurrently with emission of the optical pulse by the geodetictarget, and to capture the second image nonconcurrently with emission ofthe optical pulse by the geodetic target. The processing unit may alsobe configured to receive the first image and the second image from theat least one imaging device, to obtain a difference image between thefirst image and the second image, and to determine a direction to thegeodetic target from the geodetic instrument based on a position of theoptical pulse emitted by the geodetic target in the difference image.

According to a fourth aspect, a geodetic target configured to identifyitself to a geodetic instrument by emitting an optical pulse isprovided. The geodetic target may include an optical source that may beconfigured to emit the optical pulse. The geodetic target may furtherinclude a processing unit that may be configured to synchronize theoptical source to emit the at least one optical pulse concurrently witha capturing of a first image by the geodetic instrument, andnonconcurrently with a capturing of a second image by the geodeticinstrument.

The methods, the geodetic instrument and the geodetic target of theabove aspects are advantageous for example in that a more reliableidentification of a geodetic target in terms of unambiguity may beperformed. In addition to the optical pulse emitted by the geodetictarget, the scene to be surveyed may contain additional lightoriginating from interference sources, such as for example light fromvehicles, buildings, traffic lights or light from the sun. By obtainingthe difference image between the first image and the second image, suchinterference light may be filtered out and rejected by the geodeticinstrument, thereby reducing or eliminating the risk of the geodeticinstrument becoming confused about whether a captured light originatesfrom the geodetic target or not.

In addition, by synchronizing the emission of the optical pulse and thecapturing of the first image and the second image as described above,the optical pulse may only have to be emitted during a short time whenthe imaging device captures the first image. This may allow for theduration of the optical pulse to be reduced, and for the amplitude ofthe optical pulse to be increased while still keeping the powerconsumption of the optical source within its thermal limits. With anincreased amplitude of the optical pulse, the distance the pulse maytravel before getting too attenuated to be detected by the imagingdevice arranged at the geodetic instrument may thereby be increased,which may allow for an increased measuring distance between the geodeticinstrument and the geodetic target.

According to one embodiment, the synchronizing may include receiving asignal indicative of a common event. A signal indicative of the commonevent may for example be a radio signal, an optical signal or anelectronic signal. The signal may contain e.g. a pulse that indicatesthe common event, or the signal may be more complex and contain manypulses and/or codes which together indicate the common event. It is alsoenvisaged that the signal may be a mechanical signal, where e.g. a pulseis transferred using for example a shock wave or one or more othermechanical forces/impulses.

According to one embodiment, the signal indicative of the common eventmay include an electromagnetic signal or an electric signal. Withelectromagnetic signal, it is envisaged e.g. a radio signal, a wirelessnetwork signal, an optical signal or similar. With electric signal, itis envisaged a signal propagated within for example a cable or a wire.

According to one embodiment, the signal indicative of the common eventmay include a global navigation satellite system, GNSS, code. Such asignal may originate from e.g. a GPS, GLONASS or BeiDou satellite, andbe received by e.g. the geodetic instrument or the geodetic target andprocessed using e.g. an integrated circuit adapted for such a purpose.

According to one embodiment, the signal indicative of the common eventmay be a single electric pulse or a single electromagnetic pulse. Byusing a single pulse, the need for more complicated integrated circuitrymay be reduced.

According to one embodiment, the signal indicative of the common eventmay be received by the geodetic instrument from the geodetic target. Inone embodiment, the signal indicative of the common event may bereceived by the geodetic target from the geodetic instrument. In afurther embodiment, the signal indicative of the common event may bereceived by the geodetic target and the geodetic instrument from a thirdparty device. A third party device may for example be a satellite, aradio tower broadcasting a time signal, or other broadcasting equipment.A third party device may also be an additional geodetic instrument or anadditional geodetic target.

According to one embodiment, the first image and the second image may becaptured within a specific time interval. The specific time interval mayfor example correspond to at most one frame readout time plus two timesa frame exposure time of the at least one imaging device. By capturingthe first image and the second image close together in time, the effectof changes occurring in the scene to be surveyed may be reduced. Suchchanges may for example be something that is moving in the scene. Aspecific time interval corresponding to one frame exposure time forcapturing of the first image, one frame readout time for readout of thecaptured first image, and one frame exposure time for capturing of thesecond image may result in a minimal elapsed time between the capturingof the two images with respect to the performance of the imaging device.

A specific interval corresponding to at most two times a frame exposuretime plus one frame readout time of the at least one imaging device isonly an example providing a further benefit of the present disclosure.In certain scenarios and situations, the specific time interval may belonger.

Herein, a frame exposure time may correspond to the time taken by theimaging device to capture an image (i.e. to collect enough photons ate.g. an imaging sensor in the imaging device), while a frame read outtime may correspond to the time required to read out the imageinformation from the imaging sensor.

According to one embodiment, the first image may be captured by a firstimaging device, and the second image may be captured by a second imagingdevice. With two imaging devices, the first image and the second imagemay be captured simultaneously or within a time interval shorter thane.g. a frame readout time of one of the imaging devices. This mayfurther help to reduce the effect of changes in the scenery, such asmovement of vehicles, movement due to wind, or movements due to othercauses.

According to one embodiment, the capturing of the first image and thesecond image may be repeated to capture a plurality of first images anda plurality of second images in an interleaved fashion. Here, an“interleaved fashion” means that the images are captured such that afirst image is followed by a second image, and that this second image inturn is followed by another first image and so on in an alternatingmanner. By capturing a plurality of first images and second images,multiple difference images may be obtained and a direction to thegeodetic target from the geodetic instrument may be determined based onthe position of the optical pulse in multiple difference images. Thegeodetic target may be tracked over time, and if, for example, thegeodetic target is moving relative to the geodetic instrument, themovement of the geodetic target may be determined from the determineddirections over time.

According to one embodiment, the plurality of first images may becaptured at a rate corresponding to a rate at which the geodetic targetmay be configured to emit a plurality of subsequent optical pulses. Bycapturing the plurality of first images at the same rate at which thegeodetic target emits pulses, each pulse may be captured and thedirection to (or the position of) the geodetic target may be determinedby the geodetic instrument over time and more accurately.

According to one embodiment, the plurality of first images and secondimages may be captured at a combined rate corresponding to twice therate at which the geodetic target may be configured to emit theplurality of subsequent optical pulses. By capturing the plurality offirst images and second images at a combined rate twice that at whichthe geodetic target emits optical pulses, each first image may becaptured such that it contains an optical pulse emitted by the geodetictarget, and each second image may be captured such that it does notcontain an optical pulse emitted by the geodetic target. An image“containing” or “not containing” an optical pulse may be interpreted asthe image containing or not containing any data representative of theoptical pulse, respectively.

According to one embodiment, a time interval between capturing of afirst image and capturing of an immediately preceding second image maybe equal to a time interval between capturing the first image andcapturing of an immediately succeeding second image. Here, an image issaid to be “immediately preceding/succeeding” another image if no otherimages are captured by the geodetic instrument (using the at least oneimaging device) between the images.

According to one embodiment, the plurality of first images and secondimages may be captured at a combined rate corresponding to at least 60Hz. By capturing the first and second images at a combined rate of 60 Hzor more, the scenery to be surveyed and the geodetic target may beapproximated as static over the interval during which a first image anda second image are captured.

According to one embodiment, a frame exposure time of the at least oneimaging device may be shorter than or equal to a duration of the opticalpulse emitted by the geodetic target.

Generally, in order to capture at least a part of the emitted opticalpulse, it may be appreciated that the frame exposure at least partlyoverlaps with the optical pulse in time.

If the exposure of a frame and the emission of the optical pulse arealigned in time, a frame exposure time that is equal to the duration ofthe optical pulse may allow for a large or maximal number of photons inthe optical pulse to be captured by the imaging device without anyexposure during times when no photon is emitted.

A frame exposure time that is shorter than the duration of the opticalpulse may allow for a sufficient number of photons in the optical pulseto be captured with no exposure during times when no photon is emitted,even in the presence of a misalignment between the exposure of the frameand the emission of the optical pulse which is small enough that thefull exposure is still contained within the interval of the duration ofthe optical pulse.

According to one embodiment, a method as described above may furtherinclude synchronizing the geodetic instrument with the geodetic targetand a second geodetic target configured to identify itself by emittingan optical pulse for capturing the first image nonconcurrently withemission of the optical pulse by the second geodetic target. Bysynchronizing the geodetic target and the second geodetic target suchthat they do not emit pulses at the same time, both targets may betracked by the geodetic instrument, and directions to both targets maybe determined from multiple difference images. As the first image iscaptured while only one pulse is emitted, the geodetic instrument mayidentify the optical pulse in the first image and/or a second imagecaptured nonconcurrently with the emission of the optical pulse from thegeodetic target more easily, even in the presence of more than onegeodetic target.

According to one embodiment, a method as described above may furtherinclude capturing a third image of the geodetic target and the secondgeodetic target using at least one imaging device arranged at thegeodetic instrument. A method as described above may also includesynchronizing the geodetic instrument with the geodetic target and thesecond geodetic target for capturing the third image concurrently withemission of the optical pulse by the second geodetic target andnonconcurrently with emission of the optical pulse by the geodetictarget. A method as described above may further include obtaining asecond difference image between the third image and an image capturednonconcurrently with the emission of the optical pulse by the secondgeodetic target, and determining a direction to the second geodetictarget from the geodetic instrument based on a position of the opticalpulse emitted by the second geodetic target in the second differenceimage. As described, such a method is advantageous in that both thegeodetic target and the second geodetic target may be identified by thegeodetic instrument from different difference images. The differenceimage may be used to obtain a direction to the geodetic target, and thesecond difference image may be used to obtain a direction to the secondgeodetic target. As described above, the geodetic instrument may in thisway track multiple targets, and the capturing of the images as describedabove may help to reduce cross talk between multiple targets and allowfor more convenient identification of each target.

According to one embodiment, synchronizing the geodetic instrument withthe geodetic target as in a method described above may further includeestablishing a time reference common to the geodetic instrument and thegeodetic target, and obtaining information about allocation of a set ofnon-overlapping time slots including at least a first time slot and asecond time slot. The optical pulse may be emitted by the geodetictarget within the first time slot, and the first image and the secondimage may be captured by the geodetic instrument within the first timeslot and the second time slot, respectively. By establishing time slotsas described above, a geodetic target may be assigned a subset of timeslots (e.g. the first time slot) in which it is allowed to emit anoptical pulse. When the geodetic instrument captures images, it may takethe given allocation of time slots into account in order to capture atleast one image that will contain an optical pulse emitted by thegeodetic target, and at least one image that will not contain an opticalpulse emitted by the geodetic target, and subsequently obtain acorresponding difference image and determine a direction to the geodetictarget. To obtain information about allocation of the set ofnon-overlapping time slots, the geodetic instrument and the geodetictarget may for example be provided with information indicative of thedistribution of time slots in time, such as when a certain time slotstarts and when it ends. For a plurality of time slots, the informationmay e.g. indicate when the first time slot in the plurality of timeslots starts, how long each time slot is and how far in time they areseparated. It is envisaged that time slots belonging to the sameplurality of time slots may also be spaced unevenly in time, and theinformation indicative of the allocation may then contain the start andduration of each time slot within the plurality of time slots, or otherinformation which enables e.g. the geodetic instrument or geodetictarget to calculate the start and duration of time slots. The allocationof the time slots may be static, and the information indicative of theallocation of the time slots may be provided to the geodetic targetand/or geodetic instrument during fabrication of the devices, duringsetup before operation or during operation itself. The allocation of thetime slots may in some embodiments be dynamic, and changes of theallocation may be communicated to the geodetic target and/or geodeticinstrument when necessary. The information may be provided in connectionwith, such as before, a change of allocation, or information may beprovided from which the geodetic target and/or geodetic instrument maycalculate when a change will occur and to what extent. At longerdistances, it may for example be advantageous with longer time slots toprovide an improved integrated signal. At shorter distances, it may forexample be advantageous with shorter time slots to provide e.g. a fasterreaction time. Depending on the distance between the geodetic instrumentand the geodetic target, the length of one or many time slots may beadjusted accordingly.

According to one embodiment, synchronizing the geodetic instrument withthe geodetic target and the second geodetic target as in a methoddescribed above may include establishing a time reference common to thegeodetic instrument, the geodetic target and the second geodetic target,and obtaining information about allocation of a third time slot in theset of non-overlapping time slots. The optical pulse may be emitted bythe second geodetic target within the third time slot. By assigning asubset of time slots (e.g. the third time slot) to the second geodetictarget, where this subset of time slots does not overlap with e.g. asubset of time slots assigned to the geodetic target, the geodetictargets may both emit pulses which do not collide with each other intime.

According to one embodiment, the third image of the geodetic target andthe second geodetic target may be captured by the geodetic instrumentwithin the third time slot. By taking the assignment of time slots intoaccount, the geodetic instrument may capture at least one image thatcontains an optical pulse emitted by the geodetic target only, and atleast one image that contains an optical pulse emitted by the secondgeodetic target only. Since the assigned subsets of time slots arenon-overlapping, the geodetic target may also capture at least one imagewhich does not contain any pulse emitted by the geodetic target, and atleast one image which does not contain any pulse emitted by the secondgeodetic target. These images may be a same image, or be differentimages, as long as corresponding difference images may be obtained andfrom which directions to the geodetic target and the second geodetictarget may be determined. It is envisaged that the difference image maybe obtained from an image which contains a pulse from a geodetic targetand an image which contains no pulse from any geodetic target. It isalso envisaged that a difference image may be obtained from an imagewhich contains a pulse from a geodetic target and an image whichcontains one or more pulses from one or more different geodetic targets,as long as an optical pulse from the geodetic target to which adirection from the geodetic instrument is to be determined is notcontained in both images used to obtain the difference image.

According to one embodiment, a method as described above may includedetermining a position of the geodetic target relative to the geodeticinstrument based on the position of the optical pulse emitted by thegeodetic target in the difference image. The geodetic instrument maydetermine the position of a geodetic target if, for example, a directionto the geodetic target is known. If, for example, the geodeticinstrument is aware of its own position (e.g. by using satellitenavigation), and if the geodetic instrument is also aware of thedistance to the geodetic target (obtained by using e.g. a laser rangefinder or similar), the position of the geodetic target may becalculated by the geodetic instrument based on the determined directionto the geodetic target from the geodetic target. If the geodeticinstrument is not aware of its own position, the geodetic instrument mayhowever still at least determine the position of the geodetic targetrelative to the geodetic instrument.

According to one embodiment, a geodetic instrument as described abovemay further include a receiver configured to receive a signal indicativeof a common event, and the processing unit of such a geodetic instrumentmay be configured to synchronize the at least one imaging device basedon the received signal indicative of the common event.

According to one embodiment, the receiver may be a GNSS receiver. A GNSSreceiver may be for example a time reference module, or a common GPS,GLONASS and/or BeiDou receiver, and may be integrated into e.g. aprocessing unit or be a separate circuit that may communicate with aprocessing unit.

According to one embodiment, the at least one imaging device may be adigital camera. Using a digital camera may reduce the need for moreexpensive imaging devices.

According to one embodiment, the processing unit of a geodeticinstrument as described in the above embodiment may be configured tooperate according to a method as defined in any of the aboveembodiments.

According to one embodiment, a geodetic target as described above mayfurther include a receiver configured to receive a signal indicative ofa common event, and a processing unit of a geodetic target as describedabove may be configured to synchronize the optical source based on thereceived signal indicative of the common event.

According to one embodiment, the receiver of a geodetic target asdescribed above may be a GNSS receiver.

According to one embodiment, a geodetic surveying system may beprovided. The geodetic surveying system may include a geodeticinstrument as defined in any of the above embodiments described aboveand at least one geodetic target as defined in any of the abovedescribed embodiments.

According to one embodiment, the geodetic surveying system may include athird-party device configured to transmit a signal indicative of acommon event. The geodetic instrument and the at least one geodetictarget may be configured to receive the signal indicative of the commonevent from the third-party device. The processing unit of the geodeticinstrument may be configured to synchronize the at least one imagingdevice, and the processing unit of the geodetic target may be configuredto synchronize the at least one optical source, based on the receivedsignal indicative of the common event.

The present disclosure relates to all possible combinations of featuresrecited in the claims. Further, any embodiment described with referenceto a method according to the first aspect or the second aspect of thepresent disclosure may be combinable with any one of the embodimentsdescribed with reference to the geodetic instrument according to thethird aspect and/or the geodetic target according to the fourth aspectof the present disclosure, or vice versa.

Further objects and advantages of the various embodiments of the presentdisclosure will be described below by means of exemplifying embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments will be described below with reference to theaccompanying drawings, in which:

FIG. 1a illustrates a geodetic instrument, FIG. 1b illustrates ageodetic target, and FIG. 1c illustrates a geodetic surveying systemaccording to embodiments of the present disclosure;

FIG. 2a illustrates a method for determining a direction to a geodetictarget from a geodetic instrument according to one or more embodimentsof the present disclosure;

FIG. 2b illustrates a method for determining a direction to a geodetictarget and also to a second geodetic target from a geodetic instrumentaccording to one or more embodiments of the present disclosure; and

FIGS. 3a, 3b, 3c, 3d and 3e illustrate examples of how to capture imagesand emit optical pulses in accordance with one or more embodiments ofthe present disclosure.

In the drawings, like reference numerals will be used for like elementsunless stated otherwise. Unless explicitly stated to the contrary, thedrawings show only such elements that are necessary to illustrate theexample embodiments, while other elements, in the interest of clarity,may be omitted or merely suggested. As illustrated in the figures, thesizes of elements and regions may be exaggerated for illustrativepurposes and, thus, are provided to illustrate the general structures ofthe embodiments.

DETAILED DESCRIPTION

Exemplifying embodiments will now be described more fully hereinafterwith reference to the accompanying drawings. The drawings show currentlypreferred embodiments, but the invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided forthoroughness and completeness, and fully convey the scope of the presentdisclosure to the skilled person.

With reference to FIGS. 1a, 1b and 1c , a geodetic instrument, ageodetic target and a surveying system according to some embodiments aredescribed in the following.

FIG. 1a illustrates a geodetic instrument 110. The geodetic instrument110 may for example be a total station, a scanner or a theodolite, andmay include at least one imaging device 112 and a processing unit 116configured to carry out at least parts of one or more methods as will bedescribed later herein.

The imaging device 112 is configured to capture images within its fieldof view. The imaging device 112 may for example be a digital camera. Thedigital camera may be a still camera or a video camera. The camera mayhave a global shutter although it is also envisaged to use a camera witha rolling shutter or similar.

FIG. 1b illustrates a geodetic target 120. The geodetic target 120 mayinclude at least one optical source 124 and a processing unit 126configured to carry out at least parts of one or more methods as will bedescribed later herein.

The at least one optical source 124 may be configured to emit an opticalpulse 130, for example towards the geodetic instrument 110. The at leastone optical source 124 may for example include a light emitting diode(LED), a laser diode or any other suitable light source which may emitpulses of light in a controlled manner. Although not illustrated in FIG.1b , the geodetic target 120 is also envisaged to include drivingcircuitry for providing power to the at least one optical source 124,and connections which allows the processing unit 126 to control the atleast one optical source 124 via e.g. such driving circuitry. To powerthe at least one optical source 124, the geodetic target 120 may includealso a battery (not illustrated), although it is envisaged that powermay be provided to the geodetic target 120 also from an external sourcethrough a wire or the like. If, for example, the geodetic target 120 ismounted on a vehicle or other structure, power may be provided to thegeodetic target 120 from this vehicle or other structure.

FIG. 1c illustrates a surveying system 200 that includes a geodeticinstrument 110 and at least one geodetic target 120.

The geodetic instrument 110 and the at least one geodetic target 120 maybe synchronized with each other such that the operation of theirconstituents may be performed in a synchronized manner. For thispurpose, both the geodetic instrument 110 and the geodetic target 120may for example contain a clock. Such a clock may be a high accuracyclock having low or little drift. The clocks may be reset beforeoperation of the geodetic surveying system 200, or during operation,when the geodetic instrument 110 and the geodetic target 120 both detecta common event.

Once synchronization of the geodetic instrument 110 and the geodetictarget 120 is established, maintaining of the synchronization may becarried out by the clocks. If necessary, it is envisaged thatsynchronization may be reestablished by the sharing of another commonevent which may be detected by the geodetic instrument 110 and thegeodetic target 120 in a similar manner as described above. Ifreestablishment of synchronization is possible to be performed oftenenough, it is envisaged that the maintaining of synchronization by theclocks may not be necessary as synchronization may then be establishedor reestablished whenever needed.

To agree on the common event, the geodetic instrument 110 and thegeodetic target 120 may for example receive a signal that is indicativeof the common event. Such a signal may for example be an optical signal,a radio signal or an electronic signal, and the common event may beindicated by a pulse included in the signal. The signal may also be morecomplex and consist of several pulses and/or codes, such as for examplea global navigation satellite system (GNSS) signal. In FIG. 1c , such athird-party device is illustrated as a satellite 210, which transfers asignal 220 indicative of the common event that is received by both thegeodetic instrument 110 and the geodetic target 120. In someembodiments, it is envisaged that for example one of the geodeticinstrument 110 and the geodetic target 120 replaces the function of thethird party device, and that a signal indicative of the common event istransferred from one of the geodetic instrument 110 and the geodetictarget 120 to the other. It is also envisaged that one of the geodeticinstrument 110 and the geodetic target 120 may receive the signalindicative of the common event from a third-party device, and thenforward and/or relay this signal to the other device. If the signalindicative of the common event is a satellite signal, one or both of ageodetic instrument 110 and a geodetic target 120 may be equipped withe.g. a satellite receiving circuit and/or a satellite time referencemodule in order to receive and process the signal.

In some embodiments, the signal indicative of the common event may be awireless signal, such as a radio signal or an optical signal, and thegeodetic instrument and/or the geodetic target may be equipped with asuitable detector such as a radio antenna, a WiFi-, Bluetooth- orZigbee-detector/receiver, and/or an optical pulse detector fabricatede.g. from Si, Ge and/or InGaAs. It may, however, also be envisaged thatthe signal indicative of the common event is transferred to the geodeticinstrument 110 and/or geodetic target 120 using e.g. a fiber (throughwhich e.g. an optical pulse may be received), a cable or a wire (throughwhich e.g. an electrical signal/pulse may be received). This may beuseful if, for example, the geodetic surveying system 200 is used in anarea where reception of e.g. a radio signal from a satellite is notpossible, for example underground or in areas of dense vegetation and/orwith many and high buildings. If a fiber, cable or wire is used, thegeodetic instrument and/or the geodetic target may be equipped withsuitable connectors to which such a fiber, cable or wire may beattached.

Other methods of receiving the signal indicative of the common event mayalso be envisaged, such as for example receiving the signal as a radarpulse by the use of a suitable radar pulse receiver.

If using internal clocks, the geodetic instrument 110 and the geodetictarget 120 may be connected (by e.g. a cable) to and synchronized witheach other before operation, and then disconnected during operationitself as synchronization is then maintained by the internal clocks.

It is also envisaged that other methods of obtaining and maintainingsynchronization is also possible. Such methods may, for example, includesensing the phase of the (e.g. 50 or 60 Hz) AC signal on the main powergrid to maintain synchronization, or e.g. a broadcasted time signal froma nearby radio tower.

With reference to FIG. 2a , a method for determining a direction to ageodetic target from a geodetic instrument according to one or moreembodiments is described in more detail.

In FIG. 2a , a geodetic instrument 110 uses at least one imaging device112 arranged at the geodetic instrument 110, to capture a first image150. The first image 150 shows the scene that is to be surveyed. In thisexample, the scene contains a landscape including e.g. a lake, trees, aroad, some hills, and also structures such as a building, a radio towerand a construction vehicle. In the scene is also a geodetic target 120that emits an optical pulse 130 towards the geodetic instrument 110. Inthe captured first image 150, the optical pulse 130 emitted by thegeodetic target 120 is visible, and also other optical interferences 140such as solar reflections from the lake, light from a window in thebuilding, light from the construction vehicle and light from the radiotower.

The geodetic instrument 110 also captures, using the at least oneimaging device 112, a second image 152 covering the same scene. Thesecond image also includes the geodetic target 120. The geodeticinstrument 110 and the geodetic target 120 are synchronized, in a mannerdescribed earlier or similar, such that the optical pulse 130 is emittedby the geodetic target 120 concurrently with the capturing of the firstimage 150, and nonconcurrently with the capturing of the second image152. This is why the optical pulse 130 emitted by the geodetic target120 is visible in the first image 150 but not in the second image 152.

After having captured the first image 150 and the second image 152, thegeodetic instrument 110 obtains a difference image 160. Here, thedifference image 160 is obtained by subtracting the second image 152from the first image 150. Since the first image 150 and the second image152 are assumed to be similar except for the presence of the opticalpulse 130 in the first image 150, the difference image 160 will containno signatures or at least negligible signatures of the interferinglights. Based on the position 170 of the optical pulse 130 in thedifference image 160, the geodetic instrument 110 may determine adirection to the geodetic target 120 from the geodetic instrument 110.

With reference to FIG. 2b , a method for determining a direction to ageodetic target and also to a second geodetic target from a geodeticinstrument according to one or more embodiments is described in moredetail.

In FIG. 2b , a geodetic instrument 110 captures a first image 150 of ascene to be surveyed. Compared to the scene illustrated in FIG. 2a , thescene in FIG. 2b does not contain a construction vehicle. Instead, thescene as captured in the first image 150 contains a second geodetictarget 122 located near the lake. Except for this difference, the twoscenes in FIG. 2a and FIG. 2b are the same.

The geodetic instrument 110, the geodetic target 120 and the secondgeodetic target 122 are synchronized, in a manner described earlier orsimilar, such that the first image 150 contains an optical pulse 130emitted by the geodetic target 120 only. The geodetic instrument 110also captures a second image 152 in which no optical pulse is present.As described with reference to FIG. 2a , the geodetic instrument 110 maygenerate a difference image 160, identify the position 170 of theoptical pulse 130 in the difference image 160, and determine a directionto the geodetic target 120 from the geodetic instrument 110 based on theposition 170. As described earlier, a “position of an optical pulse inan image” may be interpreted as the position of data indicative of theoptical pulse in the image.

In addition, the geodetic instrument 110, the geodetic target 120 andthe second geodetic target 122 are synchronized such that the geodeticinstrument 110 may capture a third image 154 concurrently with emissionof an optical pulse 132 by the second geodetic target 122. The opticalpulse 132 is emitted by the second geodetic target 122 nonconcurrentlywith the emission of the optical pulse 130 by the geodetic target 120.This is why the third image 154 does not contain the optical pulse 130emitted by the geodetic target 120. The geodetic instrument 110 may alsocapture a fourth image 156 nonconcurrently with the emission of theoptical pulse 132 by the second geodetic target 122, and obtain a seconddifference image 162 by subtracting the fourth image 156 from the thirdimage 154. From the second difference image 162, the geodetic instrument110 may determine, based on a position 172 of the optical pulse 132emitted by the second geodetic target 122, a direction to the secondgeodetic target 122 from the geodetic instrument 110.

As indicated by the dashed arrow in FIG. 2b , instead of capturing thefourth image 156 the geodetic instrument may also create the seconddifference image by subtracting the second image 152 from the thirdimage 154, as long as the optical pulse 132 emitted by the secondgeodetic target 122 is not present in the image subtracted from thethird image 154. It is also envisaged that the geodetic instrument maycreate the second difference image by subtracting the first image 150from the third image 154. Also, it is envisaged that the differenceimage 160 may be created by subtracting the third image 154 from thefirst image 152. As long as a pulse from a geodetic target is presentonly in one of two images, the two images may be subtracted to extractthe position of the pulse in the resulting difference image.

With reference to FIGS. 3a -3 e, examples of how to capture images andemit optical pulses in accordance with embodiments of the presentdisclosure are described in more detail.

In FIG. 3a , the geodetic instrument 110 captures a plurality of firstimages 150 and a plurality of second images 152, and the geodetic target120 emits a plurality of optical pulses 130. The plurality of firstimages 150 and the plurality of second images 152 are captured in aninterleaved fashion, such that a capturing of a first image 150 ischronologically followed by a capturing of a second image 152 and viceversa.

The geodetic instrument 110 and the geodetic target 120 are synchronizedsuch that each optical pulse 130 is emitted concurrently with thecapturing of a first image 150, and nonconcurrently with the capturingof a second image 152. In the example, the spacing in time between eachcapture is equal for all images. A set of non-overlapping time slots,including first time slots 180 and second time slots 182 are allocatedand spaced equally, and the emission of an optical pulse 130 and acapturing of a first image 150 takes place in one of the first slots180, and a capturing of a second image 152 takes place in one of thesecond slots 182. In the example, the distances in time between e.g. asecond time slot 182 and the immediately preceding first time slot 180and the immediately succeeding first time slot 180 are equal. Phraseddifferently, the geodetic instrument 110 and the geodetic target 120 aresynchronized such that optical pulses 130 are emitted by the geodetictarget 120 with half the image capturing rate of the geodetic instrument110, such that an optical pulse 130 is present in every second capturedimage (i.e. in each first image 150). It will be appreciated thatalthough it is in this particular example shown that the time slots ofone kind are equally spaced in time, another distribution of the timeslots over time may be used as long as the geodetic instrument and thegeodetic target are informed of the allocation of the time slots. Asdescribed earlier herein, information indicative of the allocation ofthe time slots may be transferred to the geodetic instrument 110 and thegeodetic target 120. The information indicative of the allocation may betransferred using e.g. a wireless link, a wire or cable, or by any othersuitable means. The allocation of time slots may for example follow thatof a time division multiple access (TDMA) scheme.

In FIG. 3b , the first time slots 180 and the second time slots 182 arenot evenly spaced, but instead allocated such that e.g. the distance intime between a second time slot 182 and the immediately preceding firsttime slot 180 is shorter than the distance in time between a second timeslot 182 and the immediately succeeding first time slot 180. The opticalpulses 130 are emitted, and the first images 150 are captured, in thefirst time slots 180, and the second images 152 are captured in thesecond time slots 182.

In FIG. 3c , the first time slots 180 and the second time slots 182 areallocated such that e.g. the distance in time between a second time slot182 and the immediately preceding first time slot 180 is longer than thedistance in time between a second time slot 182 and the immediatelysucceeding first time slot 180. Like in FIGS. 3a and 3b , the opticalpulses 130 are emitted and the first images 150 are captured in thefirst time slots 180, and the second images 152 are captured in thesecond time slots 182.

In FIGS. 3b and 3c , the reduced distance in time between the capturingof a first image 150 and a second image 152 may allow to reduce theimpact of e.g. a moving geodetic target 120, or a non-static backgroundof the scene to be surveyed.

In FIG. 3d , a second geodetic target 122 is configured to emit aplurality of optical pulses 132. The second geodetic target 122 issynchronized with the geodetic instrument 110 and the geodetic target120 such that the optical pulses 132 emitted by the second geodetictarget 122 are nonconcurrent with the emission of the optical pulses 130by the geodetic target 120, or with the capturing of the first images150 and the second images 152 by the geodetic instrument 110. In FIG. 3d, the set of non-overlapping time slots includes a plurality of thirdtime slots 184 allocated in between each chronological pair of a firsttime slot 180 and the immediately succeeding second time slot 182. Theoptical pulses 132 are emitted by the second geodetic target 122 in thethird time slots 184, and may therefore interfere less, or not at all,with the operation of the geodetic instrument 110 and its ability totrack the optical pulses 130 emitted by the geodetic target 120. It isalso envisaged that more optical pulses, e.g. from a third geodetictarget (not shown) that are synchronized with the geodetic instrument110, the geodetic target 120 and the second geodetic target 122 may beemitted nonconcurrently in a plurality of fourth time slots allocatede.g. between each chronological pair of a third time slot 184 and theimmediately succeeding second time slot 182, or for example between eachchronological pair of a second time slot 182 and the immediatelysucceeding first time slot 182.

In FIG. 3e , the geodetic instrument 110 is configured to capture aplurality of third images 154 concurrently with the emission of opticalpulses by the second geodetic target 122 and nonconcurrently with theemission of optical pulses 130 by the geodetic target 120, and aplurality of fourth images 156 nonconcurrently with the emission of anypulse 130 or 132 by the geodetic target 120 or the second geodetictarget 122. The third images 154 are captured in the third time slots184, and the fourth images 156 are captured in a plurality of fourthtime slots 186 allocated in the set of non-overlapping time slots.

In FIGS. 3a, 3b, 3c, 3d and 3e , the images (such as images 150, 152,154 or 156) are illustrated as having extensions (or durations) in time(i.e. the width of the bars) which are longer than those of the opticalpulses (such as optical pulses 130 or 132). It will be appreciated thatthe figures are provided for illustrative purposes only, and it may beenvisaged that at least a frame exposure time for a certain image isshorter than a duration of the pulse that is to be captured. In thesefigures, an overlap between an optical pulse and an image illustratesthat the image is captured such that it contains at least part of theoptical pulse.

In this example setup, the geodetic instrument 110 may track the opticalpulses 132 emitted by the second geodetic target 122, while the opticalpulses 130 emitted by the geodetic target 120 may interfere less, or notat all, with the operation of the geodetic instrument 110.

Like described earlier with reference to e.g. FIG. 2b , it is envisagedthat the fourth images 156 may be replaced e.g. by the second images152, or by e.g. the first images 150 as long as these images are notcaptured concurrently with the emission of any optical pulse 132 by thesecond geodetic target 122.

One or more methods as described above may increase the stability oftracking of geodetic targets by a geodetic instrument and also offere.g. more flexibility in terms of measurement distance. By combiningsynchronization of the geodetic instrument and the geodetic target withobtaining a difference image from two images of the geodetic targetcaptured by the geodetic instrument, the duration of the pulse may bereduced and cross-talk between multiple geodetic targets avoided or atleast reduced. This allows for an improved way of determining adirection to one or many geodetic targets from a geodetic instrument.

Reducing the pulse duration may allow for the maximum amplitude of thepulse to be increased without surpassing the maximally allowed powerconsumption (i.e. not exceeding the thermal limit of the optical sourceused to emit the optical pulses). Increasing the amplitude of the pulsemay allow for the pulse to travel further before becoming too weak to bedetected by an imaging device, and for geodetic targets to be tracked ata longer distance. Also, by using an active geodetic target with its ownoptical source, in contrast to a passive geodetic target which relies onreflecting back light generated and sent from somewhere else, thedistance an optical pulse travels before reaching the geodeticinstrument may be halved. As the optical power per unit area may beassumed to be reduced with the square of the distance the pulse hastravelled, the active tracking distance may be up to four times longersince the optical pulse only has to travel from the geodetic target tothe geodetic instrument (and not e.g. from the geodetic instrument tothe geodetic target and then back to the geodetic instrument, as in thecase when passive targets are used).

By synchronizing the geodetic instrument and one or more geodetictargets, the confidence that the correct geodetic target is measured maybe increased. With proper synchronization, the geodetic instrument maybe confident that a pulse detected within a specific time slotoriginates from a specific target that was synchronized to emit itsoptical pulse within that specific time slot. Likewise, knowledge aboutthe allocation of time slots may help the geodetic instrument toidentify a geodetic target based on in which time frame the geodeticinstrument emitted the optical pulse.

By taking a difference image between one image captured concurrentlywith the emission of an optical pulse by a geodetic target, and oneimage captured nonconcurrently with the emission of any optical pulse bythe geodetic target, false locks due to e.g. reflections or interferinglight from other sources may be eliminated or at least partlyeliminated. An interfering light that is static enough to be present inboth images used to obtain the difference image may be cancelled out andthereby not allowed to confuse the geodetic instrument.

By synchronization, e.g. by allocation of non-overlapping time slots,the operation of the geodetic instrument may be improved even whenmultiple geodetic targets are present. The geodetic instrument maydiscriminate more accurately between multiple targets within its fieldof view, based on the timing of when an optical pulse is emitted and onwhich geodetic target that is allowed/configured to emit optical pulsesat that specific moment in time. In this way, multiple targets may betracked and/or measured by the geodetic instrument at the same time.

In the context of the present disclosure, the wording geodeticinstrument, geodetic scanner, measuring instrument, total station,survey unit or surveying instrument may be used interchangeably.

Herein, a third-party device may be for example an additional geodeticinstrument or an additional geodetic target. The third-party device mayform part of a geodetic surveying system, and be for example an externalclock or time reference combined with means for transmitting a signalindicative of a common event to the geodetic instrument and the geodetictarget. The third-party device may also be e.g. a satellite, or a radiotower broadcasting a time signal.

A processing unit may include, by way of example, a general purposeprocessor, a special purpose processor, a conventional processor, adigital signal processor (DSP), a plurality of microprocessors, one ormore microprocessors in association with a DSP core, and any other typeof integrated circuit (IC). Examples of integrated circuits may includecircuits for receiving and/or processing satellite navigation signals ortime signals, circuits for controlling and providing power to e.g.optical sources and imaging devices, and circuits for receiving and/orprocessing imaging data from imaging devices (such as graphicalprocessing units, GPUs) or similar.

A processing unit may be configured to contain instructions that, whenexecuted, causes the processing unit to, by itself and/or by directingother components that are also included, perform one or more methodsaccording to any embodiments described herein. If a method involves theoperation of several devices, such as a geodetic instrument and ageodetic target, a processing unit located in each of the devices may beresponsible for performing the parts of a method which involves thedevice in which the processing unit is located.

The steps of any method disclosed herein do not necessarily have to beperformed in the exact order disclosed, unless explicitly stated to thecontrary.

The person skilled in the art realizes that the present disclosure is byno means limited to the embodiments described above. On the contrary,many modifications and variations are possible within the scope of theappended claims.

Although features and elements are described above in particularcombinations, each feature or element may be used alone without theother features and elements or in various combinations with or withoutother features and elements.

Further, although applications of the geodetic instrument and geodetictarget has been described with reference to a geodetic surveying system,the present disclosure may be applicable to any systems or instrumentsin which a target or object has to be detected in the vicinity of such ageodetic instrument.

Additionally, variations to the disclosed embodiments can be understoodand effected by the skilled person in practicing the claimed invention,from a study of the drawings, the disclosure, and the appended claims.In the claims, the word “comprising” does not exclude other elements,and the indefinite article “a” or “an” does not exclude a plurality. Themere fact that certain features are recited in mutually differentdependent claims does not indicate that a combination of these featurescannot be used to advantage.

1. (canceled)
 2. Method implemented by a geodetic instrument fordetermining a direction to a geodetic target configured to identifyitself by emitting an optical pulse, the method comprising: capturing,using at least one imaging device arranged at the geodetic instrument, afirst image and a second image of the geodetic target; synchronizing thegeodetic instrument with the geodetic target for capturing the firstimage concurrently with emission of the optical pulse by the geodetictarget, and for capturing the second image nonconcurrently with emissionof the optical pulse by the geodetic target; obtaining a differenceimage between the first image and the second image; and determining,based on a position of the optical pulse emitted by the geodetic targetin the difference image, the direction to the geodetic target from thegeodetic instrument, wherein synchronizing the geodetic instrument withthe geodetic target includes: establishing a time reference common tothe geodetic instrument and the geodetic target and obtaininginformation about allocation of a set of non-overlapping time slotscomprising at least a first time slot and a second time slot, whereinthe first image and the second image are captured by the geodeticinstrument within the first time slot and the second time slot,respectively.
 3. The method of claim 2, wherein the synchronizingincludes receiving a signal indicative of a common event.
 4. The methodof claim 3, wherein the signal indicative of the common event comprisesat least one of an electromagnetic signal, an electric signal, a GNSScode, a single electric pulse, or a single electromagnetic pulse. 5.-6.(canceled)
 7. The method of claim 3, wherein the signal indicative ofthe common event is received by the geodetic instrument from thegeodetic target, wherein the signal indicative of the common event issent to the geodetic target from the geodetic instrument, or wherein thesignal indicative of the common event is received by the geodetic targetand the geodetic instrument from an external device. 8.-9. (canceled)10. The method of claim 2, wherein the first image and the second imageare captured within a time interval corresponding to at most one framereadout time plus two times a frame exposure time of said at least oneimaging device.
 11. The method of claim 2, wherein the first image iscaptured by a first imaging device and the second image is captured by asecond imaging device.
 12. The method of claim 2, wherein the capturingof the first image and the second image is repeated to capture aplurality of first images and a plurality of second images in aninterleaved fashion.
 13. The method of claim 12, wherein the pluralityof first images are captured at a rate corresponding to a rate at whichthe geodetic target is configured to emit a plurality of subsequentoptical pulses.
 14. The method of claim 12, wherein the plurality offirst images and second images are captured at a combined ratecorresponding to twice the rate at which the geodetic target isconfigured to emit the plurality of subsequent optical pulses.
 15. Themethod of claim 14, wherein a time interval between a first image and animmediately preceding second image is equal to a time interval betweenthe first image and an immediately succeeding second image.
 16. Themethod of claim 12, wherein the plurality of first images and secondimages are captured at a combined rate corresponding to at least 60 Hz.17. The method of claim 2, wherein a frame exposure time of said atleast one imaging device is shorter than or equal to a duration of theoptical pulse emitted by the geodetic target.
 18. The method of claim 2,further comprising: synchronizing the geodetic instrument with thegeodetic target and a second geodetic target configured to identifyitself by emitting an optical pulse for capturing the first imagenonconcurrently with emission of the optical pulse by the secondgeodetic target. 19.-22. (canceled)
 23. The method of claim 2, furthercomprising: determining, based on the position of the optical pulseemitted by the geodetic target in the difference image, a position ofthe geodetic target relative to the geodetic instrument.
 24. A geodeticinstrument, comprising: at least one imaging device configured tocapture at least a first image and a second image of a geodetic targetconfigured to identify itself by emitting an optical pulse, and aprocessing unit, configured to: synchronize the at least one imagingdevice to capture the first image concurrently with emission of theoptical pulse by the geodetic target, and to capture the second imagenonconcurrently with emission of the optical pulse by the geodetictarget, by: establishing a time reference common to the geodeticinstrument and the geodetic target and obtaining information aboutallocation of a set of non-overlapping time slots comprising at least afirst time slot and a second time slot, wherein the first image and thesecond image are captured by the geodetic instrument within the firsttime slot and the second time slot, respectively; receive, from the atleast one imaging device, the first image and the second image; obtain adifference image between the first image and the second image, anddetermine, based on a position of the optical pulse emitted by thegeodetic target in the difference image, a direction to the geodetictarget from the geodetic instrument.
 25. The geodetic instrument ofclaim 24, further comprising a receiver configured to receive a signalindicative of a common event, wherein the processing unit is configuredto synchronize the at least one imaging device based on the receivedsignal indicative of the common event.
 26. The geodetic instrument ofclaim 25, wherein the receiver is a GNSS receiver.
 27. (canceled)
 28. Ageodetic target configured to identify itself to a geodetic instrumentby emitting an optical pulse, comprising: an optical source configuredto emit the optical pulse, and a processing unit configured tosynchronize the optical source to emit the at least one optical pulseconcurrently with a capturing of a first image by the geodeticinstrument, and nonconcurrently with the capturing of a second image bythe geodetic instrument, by: establishing a time reference common to thegeodetic instrument and the geodetic target and obtaining informationabout allocation of a set of non-overlapping time slots comprising atleast a first time slot and a second time slot, wherein the opticalpulse is emitted by the geodetic target within the first time slot, andwherein the first image and the second image are captured by thegeodetic instrument within the first time slot and the second time slot,respectively.
 29. The geodetic target of claim 28, further comprising areceiver configured to receive a signal indicative of a common event,wherein the processing unit is configured to synchronize the opticalsource based on the received signal indicative of a common event. 30.The geodetic target of claim 29, wherein the receiver is a GNSSreceiver. 31.-32. (canceled)